Add some filters to the image extraction-from-post bits so people with
existing postmeta-based flows can hook in and make magic happen.

props wonderboymusic. fixes #22960.

git-svn-id: http://core.svn.wordpress.org/trunk@23821 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Merge remote-tracking branch 'upstream/master'